Communication systems are being increasingly implemented on silica optical fibers that favorably transmit in optical bands around 1300 nm and 1550 nm. In older fiber transmission systems, an optical transmitter modulates a laser emitting in one of these two bands according to an electrical data signal. At the receiving end, an optical detector converts the modulated optical signal to an electrical signal corresponding to the originally impressed data signal. Typically, the capacity of such fiber transmission systems is limited by the opto-electronics at the two ends. Systems having electronic data rates near 2.5 Gb/s are entering service, and systems at 10 Gb/s are being developed. Further significant increases in electronic speed are not anticipated for the near future.
More recent systems have nonetheless multiplied the data capacity of an optical fiber channel by means of wavelength-division multiplexing. The transmitting end includes multiple optical transmitters, each with its own laser, and the respective lasers have slightly different but well determined wavelengths. The separate optical carriers are modulated by respective data signals, and the multiple carriers are then combined (optically multiplexed) onto a single fiber. At the receiving end, the process is reversed. An optical demultiplexer separates the WDM signal into its wavelength-designated components. Separate detectors received the different components and provide separate electrical data signals. WDM systems are being fielded with four wavelengths, and even larger numbers of WDM channels may be feasible in the future.
What has been described to this point is a point-to-point WDM telecommunications system in which all the optical signals are generated and transmitted from one point and are received and detected at another point. That is, opto-electronic conversion is required at each point of a network in which signals need to be switched into or out of transmission path. Such opto-electronic circuitry is expensive.
The most frequent form of electrical multiplexing is time-division multiplexing, in which the transmitted signal is divided into a multiple time slots organized into blocks. One data signal is assigned one slot in each block, and the destination of that data signal may well differ from data signals in adjacent time slots. One of the most fundamental components of a multiplexed electrical communication network is an add/drop multiplexer (ADM). As illustrated in FIG. 1, an add/drop multiplexer 10 receives a transport signal from an input fiber 12 and transmits the transport signal to an output fiber 14. Typically, most of the multiplexed signals pass through the add/drop multiplexer 10 from the input fiber 12 to the output fiber 14 with no change. However, the add/drop multiplexer 10 has the capability to remove one or more of the multiplexed signals from the input fiber 12 and puts them on a drop line 16. Simultaneously, it puts replacement multiplexed signals from an add line 18 onto the output fiber 14.
A wavelength-division add/drop multiplexer (WADM) is greatly desired for WDM communication networks having more than two nodes between which data is transmitted and, usually, selectively switched to other nodes according to wavelength. It is possible to include complete optical-to-electrical-to-optical conversion at the WADM, but the expense is great. It is instead desired to use an all-optical WADM in which one or more wavelengths are selectively dropped and added at the node without the need to convert the optical signals on the fiber to electrical form.
Optical wavelength-selective ADMs have been fabricated by using available wavelength multiplexers and demultiplexers, such as conventional gratings or waveguide array gratings, to demultiplex all the wavelength channels onto individual fibers, using individual 2.times.2 switches on each single-wavelength fiber to configure it for pass through or add/drop, and then remultiplexing all the signals back onto a single fiber. However, the components used in this approach introduce large losses for the pass-through channels, and the 4 equipment is costly and bulky.
Ford et al. has disclosed a WADM utilizing a linear array of micro electromechanical (MEM) mirrors in "Wavelength-selectable add/drop with tilting micromirrors," Postdeadline Papers, LEOS '97, IEEE Lasers and Electro-Optics Society 1997 Annual Meeting, Nov. 10 -13, 1997, San Francisco, Calif., pp. PD2.3, 2.4. A simplified and modified view of the optics 19 of Ford et al. is shown in the schematic diagram of FIG. 2. Two ports P.sub.1, P.sub.2 provide generally parallel but separated optical paths 20, 22 incident upon a grating 24, which wavelength separates the beams 20, 22 into their respective wavelength components. In the illustration, only two wavelengths are illustrated, the one wavelength by solid lines 20.sub.1, 22.sub.1 and the other by dashed lines 20.sub.2, 22.sub.2. Importantly, the beams of different wavelengths are angularly separated while those of the same wavelength remain substantially parallel. A lens 25 focuses all the beams onto a micro-mirror array 26 comprising separately tiltable micro-mirrors 28.sub.1, 28.sub.2. In the first position of the micro-mirrors 28.sub.1, 28.sub.2, illustrated by the solid lines, they reflect light input from the first port P.sub.1 directly back to the first port P.sub.1. That is, in these first positions, the mirrors are perpendicular to the beams 20.sub.1, 20.sub.2. However, in the second position, illustrated by dotted lines, the mirrors 28.sub.1, 28.sub.2 reflect light received from the first port P.sub.1 to the second port P.sub.2. That is, in the second positions the first mirror 28.sub.1 is perpendicular to the bisector of the beams 20.sub.1, 22.sub.1, and the second mirror 28.sub.2 is perpendicular to the bisector of the beams 20.sub.2, 22.sub.2. In the second positions, the mirrors 28.sub.1, 28.sub.2 also reflect light received from the second port P.sub.2 to the first port P.sub.1.
As mentioned, there may be additional mirrors 28 for additional WDM wavelengths, and all the mirrors are separately controllable between their two positions. A tilting angle for the mirrors 28 of about 7.degree. is sufficient. The figure shows neither the collimating lenses associated with the two ports P.sub.1, P.sub.2, nor a quarter-wave plate disposed between the grating 24 and lens 25 to average out polarization effects of the grating 24, nor a folding mirror arranged in the beam for one of the ports.
Ford et al. incorporate their optics 19 into a wavelength-division add/drop multiplexer illustrated schematically in FIG. 3. The input fiber 12, the output fiber 14 and a bi-directional optical transport path 30 are connected to a first optical circulator 31 such that optical signals received from the input fiber 12 are routed to the bi-directional transport path 30 and signals received from the bi-directional transport path are routed to the output fiber 14. The other end of the bi-directional transport path 30 is connected to the first port P.sub.1 of the optics 19, and a bi-directional client path 32 is connected to the second port P.sub.2 of the optics 19. The bi-directional client path 32, the optical add line 18 and the optical drop line 16 are connected to a second optical circulator 34 such that signals received from the add line 18 are routed to the bi-directional client path 32 and signals received from the bi-directional client path 32 are routed to the drop line 16.
Considering only one wavelength, if the micro-mirror 28.sub.1 is set in its retroreflective first position, the multiplexed signal of that wavelength is routed from the input fiber 12 into the optics 19 through the first port P.sub.1 and is reflected back out the same port P.sub.1 to be thereafter routed to the output fiber 14. However, if the micro-mirror 28.sub.1 is set in its transreflective second position, the multiplexed signal of that wavelength is instead reflected in a different direction and exits the optics 19 on the second port P.sub.2, from where is it routed to the drop line 16. Simultaneously, with the micro-mirror 28.sub.1 in its second position, a signal received from the add line 18 is routed by the second circulator 34 to the second port P.sub.2 of the optics 19 and is transreflected to the first port P.sub.1. The first circulator 31 then routes the added signal to the output fiber 14.
An interesting characteristic of the WADM structure of Ford et al. is the inability of the micro-mirrors 28 to retroreflect a signal input from the ADD line 18 through the second port P.sub.2 back to the drop line 16. In fact, this is not a problem for an ADM, since an ADM is not usually designed for a connection between the add and the drop lines. However, the Ford device cannot be used as a 2.times.2 interconnect between two transport paths. An interconnect does require transmission between the ports that Ford et al. label as the add and drop ports.
I have discovered that a good way to visualize the WADM of Ford et al. is shown in FIGS. 4A and 4B, which illustrate the angular arrangements of the beam incident on or reflected from one of the micro-mirrors in the micro-mirror array 26. The beams are shown passing through a spherical surface 40 centered on the first micro-mirror 28.sub.1 and located between the micro-mirror array 26 and the lens 25. The same basic arrangement exists for all the micro-mirrors 28 so only a single wavelength-separated beam 20, 22 needs to be considered. To be precise, each beam 20, 22 represents an angular range of a conically shaped beam. FIGS. 4A and 4B show the angular relationship between the beams 20, 22 and a normal 42 (represented by a cross) of the tilting mirror 28. In the first mirror position of FIG. 4A, the mirror normal 42 is coincident with the beam 20 from the first port P.sub.1 to thereby reflect radiation received from that port directly back to that port. Whatever radiation the mirror 28 receives from the second beam 22 from the second port P.sub.2 is reflected to a spurious beam 44, marked by a dashed circle, which is lost from the system. This spurious reflection may be described as resulting from the mirror normal 42 acting as a symmetry direction for reflections of the beam 22. In the second mirror position of FIG. 4B, the mirror normal 42 falls between the two beams 20, 22. That is, the normal is coincident with the bisector of the angle between the two beams 20, 22. As a result, the light that the mirror 28 receives from the first beam 20 through the first port P.sub.1 is reflected along the second beam 22 to the second port P.sub.2. Also, the light that the mirror 28 receives from the second beam 22 through the second port P.sub.2 is reflected along the first beam 20 to the first port P.sub.1. Both these reflections can be described in terms of the mirror normal 42 being a mirror point.
The combination of movable mirrors and a grating as shown by Ford et al. has many desirable characteristics and is able to independently add and drop at the ADM any of a number of wavelengths on the transport fiber.
However, this design suffers at least two problems. It requires two circulators to separate signals going in opposite directions on the bi-directional paths 30, 32. Circulators are expensive and add loss. In addition, the experimental results presented by Ford et al. for a device with 200 GHz channel spacings show very sharply peaked channel passbands, rather the desired flat-topped passbands.
A further problem shared by Ford et al. with many types of optical add/drop circuits is that the add and drop lines as well as the input and output lines are wavelength-division multiplexed. For the near future, a WADM represents a demarcation point between a multi-wavelength optical network for transport and an electronic network or digital switch for a client interface. Hence, a WADM having a multi-wavelength add and drop lines requires additional optical multiplexing and demultiplexing on the side of the client interface. As the number of WDM wavelength channels increases, the losses associated with the splitters and combiners begin to significantly impact the system. Equipping the detectors of the receiver with wavelength filters adds to its cost and results in an inflexibility in wavelength assignment.